[Tropical] Tropical Cyclone Discussion

[EMWIN Server]

WTNT41 KNHC 212050
TCDAT1

Tropical Depression Peter Discussion Number  13
NWS National Hurricane Center Miami FL       AL162021
500 PM AST Tue Sep 21 2021

Like clockwork compared to prior days, deep convection has reignited
closer to the increasingly elongated low-level circulation of Peter.
Despite this increase in convective coverage, there does not appear
to be much if any organization to this activity, with the
upper-level cirrus taking on the appearance of a shapeless blob. In
addition, an earlier 1157 UTC ASCAT-A pass received just after the
prior advisory showed a peak wind retrieval of only 29 kt, well to
the north of the low-level circulation. Furthermore, NOAA buoy
41043, located north of the center of Peter, has been observing peak
1-minute sustained winds between 20-25 kt over the last 6 hours.
These lower winds, in combination with the lack of tropical storm
force winds observed by the earlier Air Force Reserve Reconnaissance
mission provide enough justification to downgrade Peter to a
tropical depression this advisory with maximum sustained winds of 30
kt.

The ongoing afternoon convection appears to have slowed down the
forward motion of Peter a bit this afternoon, but the heading
remains off to the west-northwest at 300/7 kt. Peter is still
expected to gradually move to the west-northwest in the short term,
followed by a somewhat sharp turn to the north and north-northeast
as a weakness in the low-level flow develops from a deep-layer
trough positioned near Bermuda. Similar to this morning, the
guidance is in general agreement on this solution, with some cross-
and along-track spread. The latest NHC track forecast is just a bit
more right compared to the previous advisory, leaning a bit closer
to the consensus aids which have also shifted a bit right this
advisory.

Peter's convective activity is unlikely to help improve the
increasingly elongated vortex, mainly because the convection is
likely to entrain dry mid-level air that often results in cool
downdrafts disrupting the low-level cyclonic circulation more than
helping. With deep-layer shear between 25-35 kt expected to persist
for the next 36 hours in both the GFS- and ECMWF-SHIPS guidance,
gradual weakening is likely to continue. By 36 hours, while
intermittent bursts of deep convection will remain possible over
warm sea-surface temperatures, both the GFS and ECMWF forecast
simulated IR brightness temperatures show the convection becoming
increasingly disorganized and unlikely to sustain Peter's status as
a tropical cyclone. Thus, the cyclone is now forecast to become a
remnant low in 36 hours. However, given that Peter's circulation is
becoming increasingly elongated, it is also possible the system may
open up into a trough even before this time period.

Key Messages:

1. Rainfall around the southern periphery of Tropical Depression
Peter could lead to areas of urban and small stream flooding through
the evening across northern Hispaniola, Puerto Rico, portions of the
Virgin Islands, and the Northern Leeward Islands.


FORECAST POSITIONS AND MAX WINDS

INIT  21/2100Z 20.5N  65.0W   30 KT  35 MPH
 12H  22/0600Z 21.3N  66.1W   30 KT  35 MPH
 24H  22/1800Z 22.3N  67.2W   25 KT  30 MPH
 36H  23/0600Z 23.2N  67.4W   25 KT  30 MPH...POST-TROP/REMNT LOW
 48H  23/1800Z 24.2N  66.9W   25 KT  30 MPH...POST-TROP/REMNT LOW
 60H  24/0600Z 25.3N  66.1W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  24/1800Z 26.7N  65.1W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  25/1800Z 28.7N  63.7W   25 KT  30 MPH...POST-TROP/REMNT LOW
120H  26/1800Z...DISSIPATED

$$
Forecaster Papin
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